When fuel injectors with solenoid drive are in operation, differing temporal opening and closing behaviors of the individual injectors, and consequently variations in the respective injection quantity, occur by reason of electrical, magnetic, mechanical and hydraulic tolerances.
The relative differences in injection quantity from injector to injector increase when the injection-times become shorter. Hitherto these relative quantitative differences were small and without practical significance. But the trend towards smaller injection quantities and shorter injection-times has the result that the influence of the relative quantitative differences can no longer be disregarded.
The temporal progression of the current intensity during a process for opening the fuel injector (in which a voltage pulse (boost voltage) is applied to the solenoid drive) is dependent on the inductance of the solenoid drive. In addition to the changing self-inductance of the solenoid drive (by reason of the non-linear ferromagnetic magnetic material), there is also a component of motional inductance by reason of the motion of the armature. The component of motional inductance begins with the beginning of the opening phase (armature/needle motion begins), and ends at the end of the opening phase (armature/needle motion ends). If the injector has been fixed, for example by mechanical clamping, in a predetermined position (for example, closed, half-open or entirely open), the current progression will likewise change by virtue of the changed inductive influences. It has turned out that it is possible to determine the instant of a predetermined opening state by analysis of one or more such current progressions (reference current progressions) and also of an determined actual temporal current progression.
The reference current progressions can be determined by measurements performed in a laboratory for a single fuel injector or for a number of fuel injectors. But such laboratory measurements are costly, provide only average values, and also cannot take into consideration changes in the behavior of the fuel injector that are due to use and that occur during the service life of the individual fuel injector.